update and fix testing to make Travis happy again

[eengstrom]

A few minor tweaks to make Travis happy again

• testing config needs some minor love to comply with the ever-changing requirements of Molecule.
• make lint happy by adding mode to installed systemd files
• disable dns-01 on Ubuntu 16.04 since we can't get latest lexicon on that platform for lack of newer Python.

Also updated the README accordingly.

resolves #24

[eengstrom]

Travis checks linked here are busted. The checks do pass:

https://travis-ci.com/github/eengstrom/ansible-role-dehydrated/builds/218362643

[eengstrom]

Ok, finally this is all green and ready for merge.

Sometimes making ansible-lint happy is a painful process.

[eengstrom]

@azielke - are you available to look at and merge this and other PRs? I have some more progress I have queued up for making this role work under Ubuntu 20.

Or, if you are done maintaining this, would you be interested letting me help? There seems to be continued interest and I'd hate to just fork and leave the community..

[eengstrom]

Sorry to be a pest, but I need to make a decision on the path to take with this role. If @azielke, you have decided to be done maintaining it, please let me know.

[azielke]

Sorry, the last year has been... eventful. I'll look into all open issues this weekend to get this project back on track.

[eengstrom]

Totally understand how the past year has been.
At any point, if you are willing to accept me as a contributor to this repo, I'm happy to help manage the evolution if you are unable. Feel free to email me to my commit author address if you want to discuss out of GitHub issues.